1. Field of the Invention
The present invention relates to a bump electrode provided on an integrated circuit device. The invention particularly relates to a bump electrode for an integrated circuit device in which a bump electrode metal is provided on an electrode film to be connected to the outside of a semiconductor chip of the integrated circuit device.
2. Discussion of the Prior Art
As the art of semiconductor integrated circuits has advanced, it has become possible to build a larger number of electronic circuits or circuit elements on a single semiconductor chip. At the same time, however, the need for a greater number of contact points on the semiconductor chip has increased. Today, it is sometimes necessary to build hundreds of contact points on a small chip of only some millimeters square.
A bump electrode is a metal electrode projecting from the surface of the chip. A large number of bump electrodes may be placed on a single chip reducing the space and time needed for the external contacts. Recently, it has become possible to arrange a number of small bump electrodes, each only some scores of micrometers across, along the peripheral edge of a chip with a small pitch of about 1.5 to 2 times as long as the size. The chip contact with the outside may be produced by a simple means such as inner lead bonding.
The present invention relates to such a bump electrode for an integrated circuit device which is particularly suitable to be connected with a mate member such as a connecting lead through a heating process such as soldering. The conventional structure and main production steps of the bump electrode will be described with reference to FIG. 3.
Referring to FIG. 3, a bump electrode will be described along with the main steps of manufacturing a bump electrode. A completed bump electrode is shown in FIG. 3(d). An integrated circuit is built on a wafer 1 as shown in FIG. 3(a). An n-type epitaxial layer 2, a p-type junction separation layer 3, and a p-type semiconductor layer 4 are shown in FIG. 3(a). The surface of wafer 1 is covered with an oxide film 5. A wiring film 6 of aluminum is disposed on oxide film 5 so that a first end of wiring film 6 is connected with a semiconductor layer 4 through a window portion of oxide film 5. A protection film 7 of nitriding silicon covers oxide film 5 and wiring film 6. A bump electrode is provided on the part of the wiring film 6 exposed through a window 7a of the protection film 7.
In the step shown in FIG. 3(b), the entire exposed surface is coated with titanium to form a lower foundation film 11 for the bump electrode. The entire exposed surface is then coated with a palladium film 12a and a gold film 12b to form an upper foundation film 12. The upper foundation film 12 is then patterned by photoetching to create the device shown in FIG. 3(b). The foundation films 11 and 12 are connected to the second end of the wiring film 6.
In the step shown in FIG. 3(c), the foundation films 11 and 12 are coated with a photo-resist film 8 and a window is opened by photo-processing through the photo-resist film 8 to expose only the upper foundation film 12. Gold as a bump electrode metal 13 is grown to a desired thickness on the upper foundation film 12 by electrolytic plating using the lower foundation film 11 as a plating electrode film. In the electrolytic plating, the lower foundation film 11 is connected with the negative terminal of a plating power source. The upper foundation film 12, within the surface of the wafer, 1 acts as a plating cathode when growing the bump electrode metal 13 on the upper foundation film 12. The bump electrode metal 13 is, thus, grown all at once. The photo-resist film 8 is then eliminated.
In order to make the completed device shown in FIG. 3(d), chemical etching is performed on the lower foundation film 11 using the upper foundation film 12 as a mask. Consequently, the lower foundation film 11 has the same pattern as the upper foundation film 12 as shown in FIG. 3(d).
A number of bump electrodes 10, each constituted by the foundation films 11 and 12, and the bump electrode metal 13 are formed a single wafer 1 with bump electrodes 10 separated from each other. By scribing and separating the wafer 1 into chips 9, flip chips of integrated circuit devices are obtained.
Referring to FIG. 3(d), a lead 20 forming an external conductor is connected to the bump electrode 10 by an inner lead bonding. Lead 20 is obtained by coating a thin copper strip having a thickness of scores of .mu.m with a junction metal coating 21 such as tin by plating. A connection can be formed by lightly pressing coating 21 against bump electrode 10 while heating coating 21 to melt coating 21.
Only two photo-process steps are performed to produce the bump electrode 10 shown in FIG. 3(d). One photo-processing step is performed to pattern the upper foundation film 12 in the step shown in FIG. 3(b), and a second photo-process step is performed to pattern the photo-resist film 8 for electrolytic plating as shown in FIG. 3(c).
If a molten metal flows onto a base portion of the conventional bump electrode when an external conductor is connected to the bump electrode, the chip is apt to suffer damage from mechanical stress produced by the solidification of the molten metal. The long-term reliability of an integrated circuit device will be greatly reduced. This problem will be described with reference to the drawings.
If the heating temperature or pressing force is unsuitable when the lead 20 is joined to the metal 13 of the bump electrode 10, a metal 21a formed by melting the junction metal coating 21 may flow along the surfaces of the bump electrode metal 13 and the foundation films 11 and 12. The flowing metal 21a will then reach the surface of the chip 9.
FIG. 4 is an enlarged diagram of the base portion of the bump electrode 10 within the circle A of FIG. 3(d). As shown in FIG. 4, a small amount of molten metal 21a may flow onto the protection film 7. Shrinking during solidification of the molten metal stresses the protection film 7 causing a crack C to appear. The crack C is too small to produce an immediate problem. However, if an integrated circuit device is used at a high temperature and a high humidity, the metal of the wiring film 6 under the protection film 7 may become corroded over time causing a pit in the wiring film 6. Eventually, the wiring film 6 will be broken or disconnected.